The present invention relates to an electromechanical apparatus and method for assuring the coplanarity of the leads of surface mounted devices to any given specification.
In the field of printed circuit board assembly, surface mounted devices are rapidly replacing devices that are mounted through holes in the printed circuit board. The advantages of surface mounted devices is that they take up less room on the board and they do not need plated through holes in the board, all of which means lower cost.
One of the more serious problems existing with surface mounted devices is that the contact points of the leads don't always lie in the same plane. Thus, when a surface mounted device lies on a printed circuit board, some of the leads are so high off the solder paste on the circuit board that they do not make connection to the circuit board when the solder is reflowed by heat. See, for example, the article entitled "The Effect of Lead Coplanarity on PLCC Solder Joint Strength," by William D. Smith, SURFACE MOUN TECHNOLOGY, June, 1986.
One present method used to check the coplanarity of surface mounted devices before they are assembled to printed circuit boards incorporates the use of vision systems. Using four cameras, one for each side, or one camera and a means for rotating the package, the vision systems can measure the relative position of the leads with respect to each other.
One disadvantage of this method is the inaccuracy in calibrating the position of each of the four cameras or the inaccuracy in rotating a package in a plane. The systems are also slow because they are measuring and comparing, and their speed is dependent on the number of leads in a device.